Unequivocal identification of intracellular aluminium adjuvant in a monocytic THP-1 cell line

Intracellular Aluminium Adjuvant

Authors:  Matthew Mold, Håkan Eriksson, Peter Siesjö, Anna Darabi, Emma Shardlow & Christopher Exley

Abstract

Aluminium-based adjuvants (ABA) are the predominant adjuvants used in human vaccinations. While a consensus is yet to be reached on the aetiology of the biological activities of ABA several studies have identified shape, crystallinity and size as critical factors affecting their adjuvanticity. In spite of recent advances, the fate of ABA following their administration remains unclear. Few if any studies have demonstrated the unequivocal presence of intracellular ABA. Herein we demonstrate for the first time the unequivocal identification of ABA within a monocytic T helper 1 (THP-1) cell line, using lumogallion as a fluorescent molecular probe for aluminium. Use of these new methods revealed that particulate ABA was only found in the cell cytoplasm. Transmission electron microscopy revealed that ABA were contained within vesicle-like structures of approximately 0.5–1 μm in diameter.

Introduction

Since their inception in the 1920s aluminium based adjuvants (ABA) have remained the predominantly used adjuvants in human vaccinations¹. ABA are aluminium salts dispersed in water to form heterogeneous suspensions or gels, of hydrated colloid particles that consist of micron-sized aggregates of 1–20 μm with primary particles in the nano-size range^{2, 3, 4}. Aluminium oxyhydroxide (AlO(OH)) adjuvants such as the licensed ABA Alhydrogel® (Brenntag Biosector, Denmark)⁵ are the most commonly used manufactured ABA in clinical vaccinations. AlO(OH)-based adjuvants are principally used owing to being the most well-defined and consistent of the clinically approved ABA and in their ability to adsorb negatively charged protein antigens from aqueous solutions at physiological pH².

When formulated in vaccine preparations, ABA both potentiate the efficacy of weak antigens and shape the resultant immune response⁶. Use of antigen only often results in weak immunopotentiation and little or no antibody production⁷. Whilst the efficacy of ABA is undisputed, a consensus is yet to be reached upon their biological activities in vivo. ABA including aluminium hydroxide preparations are used in vaccinating against diphtheria, hepatitis A and B and against human papilloma virus (HPV) to prevent cervical cancers^{6, 7}. An ever intensifying research effort is currently underway to explain the observed adjuvanticity of ABA with several studies highlighting their physicochemical properties as a key determinant. Interestingly it has been shown that nanoparticles of aluminium hydroxide of ca 200 nm elicit more potent adjuvant activities in an in vivo model of murine adjuvanticity, than larger particles in the micron-size range⁹. The increased adjuvanticity of aluminium hydroxide nanoparticles was explained by their greater ease of uptake by dendritic antigen presenting cells (APCs)⁹.

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